In integrated circuits, such as microprocessors, memories, and the like, signals may be routed for relatively long distances using transmission lines. A transmission line may be a bus, a printed circuit board trace, or other type of relatively long metal line for transporting a digital signal. Typically, a printed circuit board trace has a characteristic impedance of between 50 and 75 ohms. A driver circuit is used to provide enough current to “drive” a signal the length of the transmission line. A CMOS driver circuit commonly includes a P-channel transistor and an N-channel transistor connected in series between a positive power supply voltage terminal and a ground terminal. The gates of the transistors receive an input signal, and an output terminal of the driver circuit is located between the transistors. The P-channel transistor functions as a “pull-up” transistor, and the N-channel transistor functions as a “pull-down” transistor. The output impedance of the driver circuit should match the characteristic impedance of the transmission line in order for the driver circuit to absorb the reflected signal and prevent “ringing”. Ringing is the repeated overshooting and/or undershooting of the signal. Ringing can cause reduced noise immunity and increased time for the signal to become, and remain, valid at the far end. Impedance matching is the practice of matching the impedance of the driver and/or the load to the characteristic impedance of the transmission line to facilitate the most efficient transfer of power. Impedance matching results in quieter waveforms, better noise immunity, and improved signal timing margin.